Fiber gratings are incorporated into components that form the backbone of modern information and communications technologies, and are suitable for a wide range of applications, such as information processing and optical fiber communication systems utilizing wavelength division multiplexing (WDM). There are many different fiber grating types and configurations. For example, fiber Bragg gratings are one dimensional periodic structures that are useful in lasing, filtering and sensing applications. Various Bragg grating configurations also include chirped fiber gratings useful in chromatic dispersion compensators and apodized fiber gratings that are used to eliminate sidelobes in signal transmission spectra.
The conventional method of manufacturing fiber gratings is based on photo-induced changes of the refractive index. Extended lengths of periodic fiber are produced by moving the fiber and re-exposing it to the illumination while carefully aligning the position to be in phase with the previously written periodic modulation. The fiber core utilized in the process must be composed of specially prepared photosensitive glass, such as germanium doped silicate glass. This approach limits the length of the resulting grating and also limits the produced index contrast. Furthermore, such equipment requires perfect alignment of the lasers and exact coordination of the fiber over minute distances when it is displaced prior to being exposed again to the laser beam.
A variety of revolutionary fiber gratings based on chiral fiber structures have been developed to address the drawbacks of previously known fiber gratings as well as to offer new functionality. These fiber gratings are disclosed in a commonly assigned co-pending U.S. Patent Application entitled “Customizable Chirped Chiral Fiber Bragg Grating” as well as in commonly assigned U.S. Pat. No. 6,839,486, entitled “Chiral Fiber Grating”, U.S. Pat. No. 6,741,631, entitled “Customizable Apodized Chiral Fiber Grating Apparatus and Method”, and U.S. Pat. No. 6,925,230, entitled “Long Period Chiral Fiber Grating Apparatus”, (hereinafter individually and collectively referred to as “Chiral Fiber Patents”) all of which are hereby incorporated by reference herein in their entirety.
The Chiral Fiber Patents focused on implementation of fiber grating products in form of chiral fiber structures having double helix symmetry (which resulted in the chiral fibers having properties similar to cholesteric liquid crystals, and thus being polarization sensitive). In addition, the Chiral Fiber Patents also disclosed a single helix chiral fiber configuration for use in fiber grating applications where polarization sensitivity is not necessary.
Various advantageous techniques for fabrication of various configurations of chiral fiber gratings (for example such as disclosed in the above-incorporated Chiral Fiber Patents) via twisting (and drawing, where applicable) are disclosed in commonly assigned co-pending U.S. Patent Applications entitled “Apparatus and Method for Manufacturing Periodic Grating Optical Fibers”, “Apparatus and Method of Manufacturing Chiral Fiber Bragg Gratings”, and “Apparatus and Method for Manufacturing Helical Fiber Bragg Gratings”, (hereinafter individually and collectively referred to as “Chiral Fiber Fabrication (CFF) Patent Applications”) which are all incorporated by reference herein in their entirety.
However, one of the challenges of fabricating chiral fiber gratings in accordance with the CFF Patent Applications, is preparation of proper optical fiber preforms used in fabrication of the gratings. For example, custom optical fiber preforms may be required for optimal results. Another challenge rests in fabrication of certain types of chiral fiber gratings, for example those with properties. Such gratings require twist and drawing fabrication protocols that vary during the fabrication process, and thus cause additional stress on the fiber increasing the likelihood that a custom preform may be ruined. In addition, because preforms are pre-selected for fabrication of a chiral fiber grating with a specific helical structure (i.e., single or double), a separate fabrication process is necessary for manufacturing each type of chiral fiber gratings.
It would thus be desirable to provide an advantageous method for fabricating single and double helical structure chiral fiber gratings without using custom preforms. It would also be desirable to provide a chiral fiber grating with single helix structure waveguide end-portions and a double helix structure middle portion, having apodizing properties. It would also be desirable to provide a method of fabricating chiral fiber gratings of both single helix and double helix configuration during a single continuous fabrication process.